This invention relates to a method for controlling the entry of zebra mussels into the inlet water to electrical generating plants, water plants and other plants drawing water from rivers, lakes and streams contaminated with zebra mussels, and more particularly to a method of killing the zebra mussels by heating the inlet water prior to entry into the plants.
Dreissena Polymorpha, commonly known as zebra mussels, were unknown in the waters of the United States and Canada prior to 1988. They are indigenous to Europe and it is theorized that these prolific breeders entered the waters of Lake St. Clair when a foreign ship emptied its ballast water. Since then zebra mussels have found their way into the Great Lakes, namely Lake Erie and Lake Ontario and have already been reported in the Ohio and Tennessee river systems. A single female zebra mussel may lay some ten thousand to two million eggs per year which have a four to eight year life cycle comprising four stages: the fertilized egg stage lasting some two to three days, the veliger or larvae stage lasting some two to three weeks, the settling stage and the adult stage which begins at about twelve months. In the veliger stage the zebra mussels are a free swimming planktonic larval which may disperse for miles. It is in this stage that they are small enough to enter through the screens of a power plant such as an electrical utility generating plant and municipal and industrial water plants drawing their water supply from the contaminated rivers and lakes. In the settling stage they are visible to the eye and possess visceral threads, known as byssus, which cling to or attach to almost any solid stationary surface in areas having low velocity currents, e.g., less than approximately seven feet per second. In this stage they attempt to attach or connect onto such surfaces and if a clear surface is not available, they will climb on top of others that are so attached. In this manner they colonize on the condensers and other equipment in such plants and may form into mats or clumps up to five inches thick. The zebra mussels have few natural predators and can stay out of water for substantial periods of time, up to 14 days, merely needing moist or humid areas in which to exist.
The primary use of water in power plants and the like is to cool and condense the steam that passes through the electrical generating turbines. If the steam is not properly condensed, the plant must be shut down. The condensers are large shell and tube heat exchangers which have the cooling water inside the tubes and the steam/condensate on the shell side. Power plants using fresh water taken from the rivers and lakes have been the first to encounter the costs associated with removal of zebra mussels from intakes, piping, pumps, filters and heat exchangers since the zebra mussel clumps that form can choke down areas of the tubes, piping and filters and block the water flow. When this occurs in condenser tubes, for example, the back pressure on the turbines are increased and the overload can trip the turbines. The problem has been so severe that the Electrical Power Research Institute has formed an internal zebra mussel task force and a utility advisory group to help members cope with the zebra mussel problem and to ensure reliability of the utility plants. Extensive research and development is being conducted regarding the zebra mussel problem in an effort to provide solutions which are cost effective and environmentally acceptable.
Attempts to control the zebra mussel problem to date include: chemical treatment, primarily by injection of chlorine and bromine into the water to discourage veliger settlement within the plant; mechanical cleaning, by water blasting and scraping of the walls in the intake structures and large piping and vacuuming out the removed zebra mussels; coating the pipes with a silicone or other anti-fouling coating; use of biological molluscicide, acoustic energy and other methods including thermal methods of either recirculating the warm water from the condenser and raising its temperature which, when raised to between 95.degree. and 100.degree. F. and held for about 30 minutes, kills the zebra mussels on the screens, grates and tunnels within the plant, or by limited application of electrical resistance heating or steam heating in the piping sections. All of these known methods are either of limited value or costly, or both. For example, mechanical cleaning which provides short term control requires some four or five days for a work crew to perform and can only be done when the facility has been shut down for normal maintenance. The thermal method continually raises the water temperature and, due to environmental regulations, the hot water cannot be released back into the lakes or rivers since there is a maximum temperature at which the water may have on reentry into the lake and river system to prevent damage to existing marine life. Additionally, there are limitations on chemicals and other pollutants that may be exhausted into the rivers and lakes. Thus, the present methods of controlling the zebra mussel problem in electrical utilities are of limited value and research continues to find better methods.